Most medium size and large rotorcraft have high ratio of maximum weight to rotor disc area, such ratio is called disc loading in the aeronautical vernacular.
In order to generate the high required lift from the limited disc area, rotors of such rotorcraft turn at angular velocity (RPM) which causes the blade tips at hover at sea level to be close to 70% the speed of sound (Mach 0.7 in the aeronautical vernacular). Flying at higher altitude and/or at forward speed brings the tip speed closer to Mach 1.
As any section of a rotor travels above Mach 0.7 and approaches Mach 1, transonic flow phenomena limit the maximum section lift, create shock waves, substantially increase rotor noise level, decrease rotor efficiency and/or cause oscillatory rotor loads and rotorcraft vibrations.
To delay the onset of these negative results, some of these rotors use thin airfoils, reduced tip lift coefficient and swept back tip in the outboard 10% of the blade radius (from R90 to R100 in the rotorcraft vernacular). Additionally, to further delay the onset of these results rotorcraft with highly loaded rotors often require wider blades and/or additional blades (e.g. five blades instead of four), the former decreases rotor efficiency and both measures increase rotorcraft empty weight.
As an example, the Sikorsky UH-60 Blackhawk and S-92 have swept tip main rotor blades, and there are reports in the literature extolling the virtues of such blades in reducing noise and vibration, and improving performance. See e.g., http://www.vtol.org/pdf/airc-60.pdf; and http://www.ainonline.com/Publications/HAI/HAI—2002/HAI—02d1_stcpendingpg2.html. These and all other referenced extrinsic materials are incorporated herein by reference in their entirety.
Surprisingly, however, it appears that no one has appreciated that it would be beneficial to increase the proportion of the blade that is swept beyond about 10%. Thus, there is still a need to provide further increases in efficiency through use of blades having relatively larger swept back portions.